Recirculating reaction apparatus for continuous preparation of a polyamide

ABSTRACT

A process and an apparatus for the continuous preparation of a polyamide from polyamide forming reactants. In the process, the reactants are injected continuously into a reaction zone designed to permit rapid heating and uniform mixing. The reactants are heated and uniformly mixed in the reaction zone to form a vapor and a prepolymer. The vapor is separated from the prepolymer and may be vented through a rectifying zone to recover diamine therefrom. The prepolymer is withdrawn from the reaction zone and may be passed to a second reaction zone for further reaction to form a polymer and more vapor which is vented to atmosphere. The apparatus includes a heat exchanger and a separator; the bottom of the separator being connected to the inlet of the heat exchanger by a recirculating loop and the outlet of the heat exchanger being connected to the separator by a nozzle. Recirculation means and means to inject salt solution into the recirculating loop are also included. The recirculating means may comprise a pump or the recirculating means and the heat exchanger may be combined in a vertical thermosyphon reboiler. Control means are included for venting vapor from the separator and a rectification column may be included after the separator to recover diamine from the vapor. A flash valve or a pump and a reactor (either a liquid vapor phase reactor or a flasher) may also be included.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 392,511 filed Aug. 29, 1973,now U.S. Pat. No. 3,900,450.

BACKGROUND OF THE INVENTION

This invention relates to the production of synthetic polyamides andmore particularly to a process and apparatus for producing polyamides bythe polymerization of aqueous polyamide forming salt solutions in acontinuous process.

The synthetic linear polyamides prepared in the practice of thisinvention are of the general type described in U.S. Pat. Nos. 2,071,250;2,071,253; 2,130,523; and 2,130,948. The polymers there described arehigh molecular weight products which are generally crystalline instructure showing X-ray powder diffraction patterns in the massivestate, and which are capable of being cold drawn into fibers showing bycharacteristic X-ray patterns molecular orientation along the fiberaxis.

These polyamides, generally speaking, comprise the reaction product of apolyamide-forming composition in which the molecules are bifunctionaland contain two amide-forming groups, each of which is complementary toan amide-forming group in other molecules in said composition.

These polyamides as defined above or as otherwise identified hereinaftercan be obtained, for example, by self-polymerization ofmonoaminomonocarboxylic acids, or by reacting a diamine with a dibasiccarboxylic acid in substantially equimolecular amounts, it beingunderstood that reference herein to the amino acids, diamines, anddibasic carboxylic acids is intended to include the equivalentamide-forming derivatives thereof. Amide-forming derivatives of theamino acids include the ester, anhydride, amide, lactam, acid halide,N-formyl derivatives, carbamate, and nitrile in the presence of water.Amide forming derivatives of the dibasic carboxylic acids comprise themono- and di-ester, the anhydride, the mono- and di-amide, acid halideand the following compounds in the presence of water: nitrile,cyanocarboxylic acid, cyanoamide and cyclic imide. Amide formingderivatives of the diamines include the carbamate, N-formyl derivativeand the N,N'-diformyl derivative.

While the term polyamide is inclusive of all polymeric materials whichcontain recurring amido groups, the term nylon is now accepted as thegeneric expression for those linear polyamides which may be fabricatedinto fibers. As referred to hereinafter nylon 66 is the polyamidederived from the condensation of hexamethylene diamine and adipic acid,nylon 6 is the polyamide derived from E-caprolactam and 66/6 nyloncopolymers are interpolymers of nylon 66 and nylon 6. The possiblecombinations of diamines and dibasic acids as well as amino acidssuitable for condensation reactions is quite large, however, for thepurposes of describing the invention nylon 66 and interpolymers orcopolymers of 66/6 nylon are specifically illustrated.

Relative viscosity, as used herein, is the ratio of viscosity (incentipoises) at 25° C. of a 8.4% by weight solution of polyamide in 90%formic acid (90% by weight formic acid and 10% by weight water) to theviscosity (in centipoises) at 25° C. of the 90% formic acid alone.

Various processes for the continuous polymerization of aqueous polyamideforming salt solutions, including removal of the solvent water and thevolatile by-products (mostly water) of the condensation reaction, havebeen disclosed.

In one continuous process, U.S. Pat. No. 2,361,717 to Taylor, an aqueoussolution of the polyamide forming reactants, e.g., a diaminedicarboxylicacid salt, is supplied continuously to a reactor wherein thetemperature-pressure conditions are such that formation of steam isprevented and a major portion of the salt converted to a polymer. Theresulting reaction mass is then supplied continuously to a flash tubewherein temperatures sufficiently high for polymerization to continueare maintained with a gradual pressure reduction, thus, permitting theseparation of water from the reaction mass as steam. Finally, thereaction mass is fed continuously to a heated finisher where thepolymerization is completed to the extent desired.

In another continuous process, U.S. Pat. No. 2,689,839 to Heckert, anaqueous solution of the polyamide forming reactants is fed continuouslythrough a vented jacketed vessel wherein the temperature-pressureconditions are such that a major portion of the water is removed assteam and a portion of the salt is converted to a polymer. Heckertteaches the absence of mixing in order to provide a concentrationgradient in the salt solution between the inlet and the outlet of thejacketed vessel. The residual material from the jacketed vessel is fedcontinuously to a series of flash tubes of progressively increasingdiameter and then to a heated finisher to complete the polymerization tothe extent desired.

In other continuous processes, for example, in U.S. Pat. No. 3,218,297to Sovereign and U.S. Pat. No. 3,296,217 to Tate, most of the solventwater is evaporated from the aqueous polyamide forming salt solutionwhile the salt solution is moving as a thin annular film in a firstheated zone. The major portion of the residual salt solution from thefirst heated zone is converted to low molecular weight polymer and mostof the volatile by-products (mostly water) of the condensation reactionare evaporated from a thin annular film of the salt solution in a secondheated zone at a polyamide forming temperature. The residual materialfrom the second heated zone may then be fed to a flasher and/or afinisher to attain the desired extent of reaction.

In yet another continuous process, U.S. Pat. No. 3,185,672 to Clemo etal., a hot aqueous polyamide forming salt solution is pumped through apressure tube at a polyamide forming temperature under a pressure atleast sufficient to prevent the formation of steam and for a time suchthat approximately 23 to 44% of the salt is converted to low molecularweight polyamide. The resulting solution of partially reacted monomer isexpanded adiabatically through a narrow orifice into a heated vesselmaintained at the same temperature as the pressure tube and atsubstantially atmospheric pressure. The solvent water and the volatileby-products of the condensation reaction are vented through a rectifyingcolumn to recover diamine therefrom. The partially reacted monomer isthen removed from the heated vessel for further polymerization in otherequipment.

In these prior art processes, the removal of the major portion of thesolvent water and the conversion of the major portion of the polyamideforming aqueous salt solution to low molecular weight polymer with theremoval therefrom of the major portion of volatile by-products (mostlywater) formed in the condensation reaction are carried out either in anunagitated zone or in separate zones with the result that expensive heattransfer facilities are required.

Surprisingly, it has now been found that the above disadvantages of theprior art processes may be overcome and the above steps carried outsimultaneously, in one zone, in inexpensive facilities, by injecting apolyamide forming aqueous salt solution into a reaction zone havingmixing and heating means adapted to achieve uniform mixing and rapidheat transfer; the reaction zone being maintained at a temperature andpressure adapted to allow simultaneously, the removal of solvent waterand the polymerization of the major portion of the salt.

SUMMARY OF THE INVENTION

In accordance with the present invention therefore, there is provided aprocess for the continuous preparation of a polyamide from polyamideforming reactants comprising:

injecting continuously polyamide forming reactants into a reaction zonedesigned to permit rapid heating and uniform mixing;

heating and uniformly mixing the reactants within the reaction zone fora predetermined hold-up time and at a predetermined elevated temperatureand elevated pressure to form a vapor and a prepolymer;

continuously separating the vapor from the prepolymer so formed; and

withdrawing the prepolymer from the reaction zone.

One embodiment of the present invention provides a process for thecontinuous preparation of a polyamide from a polyamide forming aqueoussalt solution prepared from a diamine and a dicarboxylic acidcomprising:

injecting continuously a polyamide forming aqueous salt solution into afirst reaction zone designed to permit rapid heating and uniform mixing;

heating and uniformly mixing the salt solution within the first reactionzone for a predetermined hold-up time and at a predetermined elevatedtemperature and pressure to form a first vapor and a prepolymer;

continuously venting the first vapor through a rectifying zone;

treating the first vapor in the rectifying zone to recover from thevapor, diamine separated from the salt solution in the first reactionzone;

passing the prepolymer from the first reaction zone into a secondreaction zone;

heating the prepolymer within the second reaction zone for apredetermined hold-up time to form a second vapor and a polymer;

continuously separating the second vapor from the polymer so formed; and

withdrawing the polymer from the second reaction zone.

The term "prepolymer" as used herein refers to the partially reactedpolyamide forming salt solution in the first reaction zone having anextent of reaction in the range of from 70 to 98 percent and a relativeviscosity in the range of about 2.3 to 11. Extent of reaction is thefraction, expressed herein as a percentage, of the reactive functionalgroups on the original reactants which have entered into reaction.

The present invention also provides an apparatus for the continuouspreparation of a polyamide comprising:

a heat exchanger having an inlet and an outlet;

a separator having mid and bottom portions;

a recirculating loop connected between the bottom of the separator andthe inlet of the heat exchanger;

a nozzle connected between the outlet of the heat exchanger and the midportion of the separator;

an injecting means in the recirculating loop adapted to inject a flow ofpolyamide forming reactants into the recirculating loop;

a heating means in the heat exchanger adapted to partially vaporize andpartially react the reactants to form a prepolymer and a vapor;

a recirculating means adapted to recirculate the prepolymer from thebottom of the separator through the heat exchanger and back into theseparator;

control means for maintaining a constant level of prepolymer in theseparator and for withdrawing a controlled flow of prepolymer therefrom;

control means for the heating means in the heat exchanger adapted tomaintain the prepolymer in the separator at a predetermined temperature;and

control means for venting the vapor from the separator and formaintaining a predetermined pressure therein.

Another embodiment of the present invention provides an apparatus forthe continuous preparation of a polyamide from a polyamide formingaqueous salt solution prepared from a diamine and a dicarboxylic acidcomprising:

a heat exchanger having an inlet and an outlet;

a rectification column having a base-section, a top portion, and anentrance for receiving a flow of polyamide forming aqueous saltsolution;

a separator comprising the base-section of the rectification column,said separator having a bottom portion and a mid portion;

a recirculating loop connected between the bottom of the separator andthe inlet of the heat exchanger;

a vapor pipe connected between the outlet of the heat exchanger and themid portion of the separator;

reflux means located within the top portion of the rectification columnadapted to supply reflux to the rectification column;

collecting means located above the separator in the rectification columnadapted to collect the salt solution and reflux from the bottom of therectification column;

an injecting means in the recirculating loop adapted to inject the saltsolution and reflux from the collecting means into the recirculatingloop;

a heating means in the heat exchanger adapted to partially vaporize andpartially react the salt solution to form a prepolymer and a vapor;

a recirculating means adapted to recirculate the prepolymer from thebottom of the separator through the heat exchanger and back into theseparator;

control means for maintaining a constant level of prepolymer in theseparator and for withdrawing a controlled flow of prepolymer therefrom;

control means for the heating means in the heat exchanger adapted tomaintain the prepolymer in the separator at a predetermined temperature;and

control means for venting the vapor from the separator and formaintaining a predetermined pressure therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a device suitable for carrying outone embodiment of the process of the present invention.

FIG. 2 is a schematic diagram showing a variation in part of the deviceof FIG. 1;

FIG. 3 is a schematic diagram showing another variation in part of thedevice of FIG. 1; and

FIG. 4 is a schematic diagram showing yet another variation in part ofthe device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring generally to FIG. 1 of the drawings, the continuouspolymerization system includes a rectifying zone 10; a first reactionzone 12 in which a polyamide forming aqueous salt solution, for example,an aqueous solution of hexamethylene diammonium adipate, is uniformlymixed, partially vaporized and partially reacted under predeterminedpressure temperature conditions; and a second reaction zone 14 whereinreaction continues at a lower pressure than the first reaction zone 12.The vapor from the first reaction zone vents through the rectifying zone10. The polymer issuing from the second reaction zone 14 passes to afinisher 64 and from there through a transfer line 68 for furtherprocessing. For example, the polymer may be fed to polymer pelletizing,fiber spinning or film extruding units.

Referring now to FIG. 1 in detail, a combination partialcondenser-preheater 16 is shown in the upper portion of the rectifyingzone 10. Partial condenser-preheater 16 has an inlet line 18 and a feedline 20 which connects to a mid section of a rectification column 22.The rectification column 22 has eight bubble cap trays, numbered fromthe bottom tray as T1 through T8. A collection pan 24 is positionedbelow tray 1. A liquid line 26 leads from collection pan 24 to arecirculating loop 28. The recirculating loop 28 connects the bottom ofa separator 30 comprising the base section of the rectification column22, to the bottom of a vertical thermosyphon reboiler 32. A vapor pipe34 connects reboiler 32 at its upper end to a mid section of separator30. An entrance line 36 with a throttle valve 38 is provided to supplyheating fluid, preferably Dowtherm A for heating purposes. An exit line40 is provided in the lower shell of reboiler 32 for removing heatingfluid. A vent line 44 with a control valve 46 is provided in the top ofrectifying zone 10 and an entrance line 42 is provided in column 22above tray T8. An exit line 48 having a control valve 50 connects thebottom of separator 30 through a heated entrance line 52 (or a heatexchanger) to a boiling reactor 54 (i.e., a second reaction zone 14). Avent line 56 with a control 58 is provided in the top of boiling reactor54. An outlet line 60 having a control valve 62 connects boiling reactor54 to a polymer finisher 64.

In operation, a predetermined flow of a polyamide forming aqueous saltsolution, preferably a 40 to 60% by weight solution of a polyamideforming salt such as hexamethylene diammonium adipate, is fed to thecombination partial condenser-preheater 16 through inlet line 18. Thetemperature of the solution entering the partial condenser-preheater 16is normally in the range of 25° to 50° C. and the solution is heated inthe partial condenser-preheater 16 to a temperature which may approachthe temperature of the vapor in the upper portion of the rectifying zone10. The temperature of this vapor is approximately equal to the boilingpoint of water at the pressure being maintained in the rectifying zone10. The rectifying zone 10 and the first reaction zone 12 are maintainedat a predetermined pressure, preferably in the range of from about 160to about 350 lb./sq. in. absolute. The partial condenser-preheater 16also condenses a portion of the vapor in the upper portion of therectifying zone 10 to provide reflux for the rectifying zone 10. Fromthe partial condenser-preheater 16, the solution is fed through line 20to a mid section of the rectification column 22. In the embodimentshown, the rectification column 22 has eight bubble cap trays and thepreheated salt solution is fed on tray T3, the third tray from thebottom. The number of trays, T, may be more or less than eight and thefeed line 20 may enter the column at any tray above the bottom tray andbelow the top tray. It will be appreciated that sieve plate trays orcolumn packing may be substituted for the bubble cap trays.

On entering the rectification column 22, the salt solution mixes withreflux and passes down the column from tray to tray to be collected atthe collection pan 24. The salt solution, diluted by the reflux, flowsfrom the collection pan 24 through the liquid line 26 and is injected orsparged into the recirculating loop 28.

The salt solution diluted by the reflux is injected into therecirculating loop 28 directly beneath the reboiler 32. The saltsolution mixes with the liquid circulating in the recirculating loop 28and passes into the bottom of reboiler 32. In the reboiler 32, the saltsolution is heated, partially vaporized and partially reacted to form afirst vapor and a prepolymer, which are conducted from the top of thereboiler 32 into the separator 30 by the vapor pipe 34. The vapor risesfrom the separator 30 through the rectification column 22 in which thevapor is contacted with reflux and salt solution. The vapor rising fromthe top tray T8 contacts the partial condenser-preheater 16 and ispartially condensed to produce reflux. The quantity of reflux returnedto the rectification column 22 from the partial condenser-preheater 16is governed by: the amount, concentration and temperature of the saltsolution entering partial condenser-preheater 16; the pressure in therectifying zone 10; and the temperature of the prepolymer in theseparator 30, which forms the base section of column 22. The heattransfer area of the partial condenser-preheater 16 is designed so thatany increase in the flow of solution increases the amount of vaporcondensed as reflux. The partial condenser-preheater 16 is preferablyoverdesigned. The above feature automatically keeps the reflux to feedratio in balance despite changes in feed rate of salt solution to thesystem. Thus, the polymerization system is less susceptable to upsetsdue to changes in the flow of salt solution feed than are the prior artsystems. Reflux, optionally in the form of demineralized water, may alsobe added through entrance line 42 above tray T8. If desired, all refluxmay be supplied as demineralized water. In this case, the partialcondenser-preheater 16 would be eliminated and other means forpreheating the salt solution provided if desired.

The uncondensed vapor is vented from the top of column 22 through thevent line 44 by the central valve 46 which also maintains the desiredpressure throughout the rectifying zone 10 and the first reaction zone12.

The salt solution diluted by the reflux is injected or sparged into therecirculating solution, preferably just prior to where the recirculatingloop 28 connects to the bottom of the thermosyphon reboiler 32. In somecases, additives such as titanium dioxide may be required in the polymerand preferably these are added at the point where the line 26 joins therecirculating loop 28. Feeding the salt solution into the recirculatingloop 28 at this point is important in order to promote the mixingnecessary to ensure the uniformity of the prepolymer in the firstreaction zone 12.

Some or all of the preheated salt solution may be injected directly intothe recirculation loop 28 without the solution first passing through therectification column 22. In the case where all of the preheated saltsolution is injected directly into the recirculation loop 28, thecollection pan 24 may be omitted from the rectification column 22 andthe reflux and recovered diamine may be allowed to flow from tray T1into the separator 30. It is feasible to eliminate the rectifying zone10 entirely. In this case, the vapor rising from the separator 30 isvented directly to atmosphere. The additional diamine lost from theseparator 30 may be compensated for by adding an excess of diamine inthe preparation of the salt solution. However, the uniformity of theresultant polymer usually is not as good as it is where a rectifyingzone 10 is provided above the first reaction zone 12. Moreover, theadditional diamine lost from the separator 30 adds to the raw materialscost for the polymer and may also result in a pollution problem.

In the recirculating loop 28, the salt solution mixes with the liquidrecirculating therethrough and passes into the bottom of the verticalthermosyphon reboiler 32. In the reboiler 32, the salt solution isheated, partially vaporized and partially reacted to form a first vaporand a prepolymer, which are conducted from the top of the reboiler 32into separator 30 by the vapor pipe 34. It is the formation of vaporwithin the tubes of the vertical thermosyphon reboiler 32 that causesthe circulation of the prepolymer from the separator 30 through thereboiler 32 and back into the separator. A uniform recirculation ratemay be maintained by controlling the liquid level in the separator 30. Apump or other forced circulation means, however, may be inserted in therecirculating loop 28 to circulate the prepolymer from the separator 30through a heat exchanger and back into the separator. In this case, thesalt solution is preferably injected into the recirculating loop 28after the pump.

The prepolymer in the separator 30 (in the first reaction zone 12) ismaintained at a predetermined temperature by adjusting the throttlingvalve 38 in the heating fluid entrance line 36. As indicated below inTable 1, prepolymers having extents of reaction and relative viscositiesover a considerable range are obtainable depending on the operatingconditions selected. The values indicated in Table 1 are for the case inwhich the aqueous salt solution is an aqueous solution of hexamethylenediammonium adipate.

                  TABLE 1                                                         ______________________________________                                        Operating Conditions                                                                         1         2         3                                          ______________________________________                                        Temperature Range,                                                             ° C.   215-290   220-260   220-245                                    Hold-up Time Range,                                                            minutes        15-400    40-200    50-150                                    First Reaction Zone                                                            12 Pressure Range,                                                            lb./sq. in. absolute                                                                        160-500   160-350   160-265                                    Prepolymer                                                                    Extent of Reaction,                                                            Percent       70-98     75-97       80-96.4                                  Relative Viscosity                                                             (Approx.)     2.3-11    2.4-7     2.5-6                                      ______________________________________                                    

From the bottom of the separator 30, the prepolymer is conducted by theexit line 48 to the control valve 50, which flashes it through theheated entrance line 52 (or a heat exchanger) to the boiling reactor 54,preferably a partially filled reactor, (i.e., to second reaction zone14). The prepolymer is heated to a temperature approaching that of theboiling reactor 54 by the heated entrance line 52. A jacket heated witha heating fluid such as Dowtherm A surrounds the boiling reactor 54 andmaintains a predetermined temperature in the boiling reactor 54. In theboiling reactor, the prepolymer undergoes further conversion to a secondvapor and a polymer. The second vapor is vented, preferably toatmosphere through the vent line 56 by the control valve 58 which alsomaintains a predetermined pressure in the boiling reactor 54 (secondreaction zone 14). As indicated below in Table 2, polymers havingextents of reaction and relative viscosities over a considerable rangeare obtainable depending on the operating conditions selected. Thevalues indicated in Table 2 are for the case in which the aqueous saltsolution is an aqueous solution of hexamethylene diammonium adipate.

                  TABLE 2                                                         ______________________________________                                        Operating Conditions                                                                           1           2                                                ______________________________________                                        Temperature Range,                                                             ° C.     260-300     270-290                                          Hold-up Time,                                                                  minutes          5-60       10-25                                            Pressure, lb./sq. in.                                                          absolute         30-350      50-250                                          Polymer                                                                       Extent of Reaction,                                                            Percent         96.5-99       97-98.5                                        Relative Viscosity                                                             (Approx.)        6-26        8-15                                            ______________________________________                                    

The polymer is flashed from the boiling reactor 54 through the outletline 60 and into finisher 64 by control valve 62. The pressure infinisher 64, which is lower than the pressure in the boiling reactor 54,may be at atmospheric pressure or may even be under a vacuum. Thefinisher 64 may be of the type shown by Li in U.S. Pat. No. 3,113,843,issued Dec. 10, 1963, it may be of the type disclosed by Pinney inCanadian patent application Ser. No. 114,562, filed June 1, 1971, or itmay be of other types well known in the art. The polymer leaving thefinisher 64 passes through transfer line 68 for further processing.

FIG. 2 shows a variation in the facilities of FIG. 1, wherein anoverflow weir 31 is provided in the separator 30. The overflow weir 31divides the separator 30 into two zones: a recirculating zone on therecirculating loop 28 side of the overflow weir 31 and a zone on theexit line 48 side of the overflow weir 31 in which the flow ofprepolymer approaches "plug flow". The overflow weir 31 has twofunctions:

1. It provides a constant level in the recirculating zone, therefore,variations in level in the plug flow zone have no effect on thecirculating rate through the recirculating loop and thermosyphonreboiler; and

2. It provides a reactor wherein the polymerization reaction is allowedto more closely approach equilibrium.

FIG. 3 shows another variation in the facilities of FIG. 1, wherein aflasher 53 is substituted for the boiling reactor 54 of FIG. 1. Theflasher 53 may be a tubular reaction vessel of progressively increasingdiameter inside of a jacket which is heated with a heating fluid such asDowtherm A. From the bottom of the separator 30, the prepolymer isconducted by the exit line 48 to the control valve 50, through which itis flashed via an entrance line 51 to flasher 53. It will be appreciatedthat a pump may be substituted for the control valve 50 to boost thepressure if the flasher 53 is designed to have a high pressure dropacross it. An outlet line 59 conducts the polymer and the second vaporfrom the outlet of the flasher 53 into the polymer finisher 64. Theentrance line 51 and the outlet line 59 to and from the flasher 53 arepreferably as short as possible. A vent line 65 conducts the secondvapor from the top of the finisher 64 to atmosphere or, if desired, to asource of vacuum. The polymer is conducted from the finisher asdescribed heretofore.

FIG. 4 shows yet another variation in the facilities of FIG. 1, whereinthe overflow weir 31 in the separator 30 (shown in FIG. 2 and describedabove) is combined with the flasher 53 (shown in FIG. 3 and describedabove). In certain instances, the flasher 53 may be omitted from thefacilities of FIG. 4 and the prepolymer may be conducted from the bottomof separator 30 and flashed through a heated entrance line directly intofinisher 64.

In the embodiments of the present invention shown in the drawings anddescribed above, the first reaction zone is depicted as a zone in whichrapid heating and uniform mixing is achieved by the recirculation ofprepolymer from a separator through a heat exchanger and back into theseparator. Recirculation of a prepolymer occurs either (1) because theheat exchanger is a thermosyphon reboiler; or (2) because a pump isinstalled in the recirculating loop between the separator and thereboiler. In each of the above cases, the aqueous polyamide forming saltsolution is fed to the first reaction zone at a position designed topromote rapid mixing of the salt solution with the recirculatingprepolymer. It is feasible to use other apparatus as a first reactionzone. The essential features required in such apparatus are as follows:

1. it must be designed to allow rapid heating and uniform mixing ofprepolymer within the reaction zone; and

2. it must be designed so that the aqueous polyamide forming saltsolution is fed into the first reaction zone at a position such thatrapid mixing of the salt solution with the prepolymer is achieved.

The following examples illustrate the present invention without limitingits scope:

EXAMPLE I

A nylon polymer was prepared from a 50.5% by weight aqueous solution ofhexamethylene diammonium adipate (containing appropriate textileadditives) in a polymerization apparatus as hereinbefore described andillustrated in FIG. 1.

The 50.5% by weight aqueous solution of hexamethylene diammonium adipate(salt solution) was fed through the entrance line 18 to the partialcondenser-preheater 16 at the rate of 151.6 lb./hr. and was preheatedtherein to 196° C. by condensing 59.5 lb./hr. of vapor rising from thetop tray of a six-tray (bubble cap) rectification column 22 as reflux.The preheated salt solution was fed through the line 20 on the 3rd trayfrom the bottom of the rectification column 22. The salt solution andreflux was collected on the collection tray 24 and passed by gravitythrough the line 26 and injected (as an approx. 36% by weight aqueoussolution) into the recirculating loop 28 beneath the verticalthermosyphon reboiler 32. A delusterant was injected into the saltsolution and reflux just prior to its entry into the recirculating loop28.

The pressure in the rectifying zone 10 and first reaction zone 12 wasmaintained at 225 lb./sq. in. absolute by the control valve 46, thetemperature in the separator 30 was maintained at 228° C. by adjustingthe flow of heating fluid, Dowtherm A, using throttling valve 38 and theaverage hold-up time in the separator 30 was 53.4 minutes. Theprepolymer leaving the separator 30 by exit line 48 had approximatelythe following characteristics:

1. a relative viscosity of 4.5;

2. a water content of 10% by weight; and

3. an extent of reaction of 91%.

The vapor leaving the top of the separator 30 (and entering therectification column 22) contained 1.46% hexamethylene diamine, and thevapor venting from vent line 44 contained only 0.14% hexamethylenediamine.

The prepolymer leaving the separator 30 through the exit line 48 was letdown by the control valve 50 through the heated entrance line 52 intothe horizontal boiling reactor 54 which was maintained at 120 lb./sq.in. absolute by the control valve 58 in the vent line 56. Thetemperature of the prepolymer was raised to 272° C. in the heatedentrance line 52. After 29 minutes residence (hold-up) time in theboiling reactor 54, the polymer had the following characteristics:

1. relative viscosity of 9.9; and

2. a water content of about 1.7% by weight.

The vapor from vent line 56 contained 1.13% hexamethylene diamine. Theconcentration of unreacted amine groups in the polymer was approximately44 gram equivalents per million grams of mixture less than theconcentration of unreacted carboxyl groups.

The polymer from the boiling reactor 54 was flashed through the outletline 60 by control valve 62 into the finisher 64 operated at a pressureof 800 mm. of Hg absolute. The polymer leaving the finisher 64 throughtransfer line 68 had a relative viscosity of 41.6. Particulate nylon 66resin was produced at the rate of 66.2 lb./hr. from the polymer and meltspun to produce a textile yarn of high quality and uniformity.

EXAMPLE II

A nylon polymer was prepared from 50.5% by weight aqueous solution ofhexamethylene diammonium adipate as hereinbefore described andillustrated in FIGS. 1 and 3. FIG. 3 illustrates modifications made inthe second reaction zone 14 of FIG. 1.

In the rectifying zone 10 and the first reaction zone 12, theconfiguration of the equipment and the process conditions were the sameas in Example I except that the temperature in the separator 30 was 226°C. instead of 228° C., and that the average hold-up time in theseparator 30 was 57 minutes instead of 53.4 minutes. The prepolymerleaving the separator 30 by the exit line 48 had approximately thefollowing characteristics:

1. a relative viscosity of 4.5;

2. a water content of 10% by weight; and

3. an extent of reaction of 91%.

The prepolymer leaving the separator 30, through the exit line 48, waslet down by the control valve 50 through the entrance line 51 to theflasher 53. The residence time (hold-up time) in the flasher 53 was 19minutes, the pressure drop through the flasher 53 was 68 lb./sq. in. andthe temperature of the polymer at the end of the flasher 53 was 274° C.This polymer, which was then fed to the polymer finisher 64 of ExampleI, had a water concentration of about 1.7%. This water concentration wasthe same as the concentration in the polymer leaving the boiling reactor54 in Example I.

EXAMPLE III

A nylon polymer was prepared from a 50.5% by weight solution ofhexamethylene diammonium adipate as hereinbefore described andillustrated in FIGS. 1 and 4. FIG. 4 illustrates modifications made inthe separator 30 and in the second reaction 14 of FIG. 1.

The 50.5% by weight salt solution was fed through the entrance line 18to the partial condenser-preheater 16 at the rate of 140 lb./hr. and waspreheated therein to 205° C. by condensing 56 lb./hr. of vapor risingfrom the top tray of a 12-tray bubble cap rectification column 22 asreflux. The preheated salt solution was fed through the line 20 onto the7th tray from the bottom of the rectification column 22. The saltsolution and reflux was collected on the collection pan 24 and passed bygravity through the line 26 and injected at approximately a 38%concentration into the recirculating loop 28 beneath the verticalthermosyphon reboiler 32.

Overflow weir 31 (see FIG. 4) divides separator 30 into two zones: arecirculating zone on the recirculating loop 28 side of the overflowweir 31 and a plug flow zone on the exit line 48 side of the overflowweir 31.

The pressure in the rectifying zone 10 and first reaction zone 12 wasmaintained at 265 lb./sq. in. absolute by the central valve 46, thetemperature in the separator 30 was maintained at 230° C. by adjustingthe flow of heating fluid, Dowtherm A, using throttling valve 38 and theaverage hold-up time in the recirculating zone side of the separator 30was approximately 60 minutes. Prepolymer having an extent of reaction of92.8%, a relative viscosity of about 3.6 and a concentration of 89%flowed over the overflow weir 31 into the plug flow zone of separator30. The hold-up time in the plug flow zone was 20 minutes. The polymerleaving the separator 30 by the exit line 48 had the followingcharacteristics:

1. an extent of reaction of 95.9%;

2. a relative viscosity of about 5.8; and

3. a water content of 11% by weight.

The polymer leaving the separator 30 through exit line 48 was let downby the control valve 50 through the entrance line 51 into the flasher53. The hold-up time in the flasher 53 was 23 minutes, the pressure dropthrough the flasher 53 was 180 lb./sq. in. and the temperature of thepolymer at the exit end of the flasher was 280° C. This polymer, whichwas then fed to the polymer finisher 64 of Example I, had a relativeviscosity of about 10.

The polymer produced from this process was of excellent quality.Whiteness of polymer due to a low level of degradation by-products wasparticularly obvious.

EXAMPLE IV

A nylon polymer was prepared from a 50.5% by weight solution ofhexamethylene ammonium adipate as hereinbefore described and illustratedin FIGS. 1 and 4 and from molten E-caprolactam.

Operating conditions were similar to Example III except that 71.9lb./hr. of 50.5% weight salt solution was fed through entrance line 18,reflux vapor rate was proportionately reduced and 9.1 lb./hr. of moltenE-caprolactam at about 230° C. was injected through line 26 into therecirculating loop 28.

Pressure in the rectifying zone 10 and reaction zone 12 was 250 psia andtemperature in the separator 30 was 230° C. average hold-up time in therecirculating zone side of the separator 30 was about 103 minutes and inthe plug flow about 35 minutes.

Hold-up time in the flasher 53 was 32 minutes and the temperature at theexit end was 278° C. Hold-up time in the finisher 64 was about 45minutes at a pressure of 220 torr and at 278° to 283° and in thedischarge transfer line 68 at 284° C. for 17 minutes.

The product was a random 66/6 nylon copolymer having a relativeviscosity of 53.

EXAMPLE V

The operating conditions of Example IV were maintained without changeexcept that the E-caprolactam was pumped in through entrance line 18along with hexamethylene ammonium adipate solution.

Random 66/6 nylon copolymer having a relative viscosity varying from 52to 58 was produced. It had a melt point of 220° C. and physicalproperties the same as those of similar copolymers made in batchautoclaves.

EXAMPLE VI

The operating conditions of Example IV were maintained without changeexcept that the E-caprolactam was pumped in at the mid-point of flasher53 and finisher 64 operating pressure was 320 torr.

The product was a 66/6 nylon block copolymer containing 10% by weightE-caprolactam. Its melt point was 259° C., relative viscosity was 51 andphysical properties were very similar to 66 nylon. A material balance onthe E-caprolactam injected shows that about half of it was lost as vaporthrough the finisher vent line 65.

What is claimed is:
 1. Apparatus for the continuous preparation of apolyamide from a polyamide forming aqueous salt solution prepared from adiamine and a dicarboxylic acid comprising:rectification column having abase section, a top portion, and a mid-section; a combination partialcondenser-preheater which is located within said top portion and has aninlet line for the polyamide forming aqueous salt solution and a linefor feeding the salt solution from the condenser-preheater to the columnmid-section, and which is for condensing a portion of vapor in the topportion of the rectification column for reflux and for simultaneouslypreheating the polyamide forming aqueous salt solution to be fed to saidmid-section; a separator comprising the base section of therectification column, said separator having a bottom portion and amidportion; a vertical thermosyphon reboiler having a bottom and anupper end; a recirculating loop connecting the bottom of the separatorto the bottom of the reboiler; a vapor pipe connecting the reboiler atits upper end to the midportion of the separator; collecting meanslocated in the column above the separator for collecting salt solutionmixed with reflux from the mid-section of the rectification column;injecting means for injecting the salt solution and reflux from thecollecting means into the recirculating loop connecting the bottom ofthe separator to the bottom of the reboiler; a vent line in the topportion of the rectification column for venting uncondensed vapor;control means for venting uncondensed vapor from the separator throughthe vent line in the top portion of the rectification column and formaintaining a predetermined pressure therein; an exit line connected tothe bottom portion of the separator; control means for maintaining aconstant level of prepolymer in the separator and for withdrawing acontrolled flow of prepolymer through the exit line; heating means inthe reboiler for partially vaporizing and partially reacting the saltsolution to form a prepolymer and a vapor; and control means for theheating means in the reboiler for maintaining the prepolymer in theseparator at a predetermined temperature.
 2. The apparatus of claim 1including in the separator an overflow weir which divides the separatorinto a recirculating section on the recirculating loop side of theoverflow weir and a plug flow section on the exit line side of the weir.3. The apparatus of claim 1 wherein the rectification column has atleast three bubble cap trays including a top tray and a bottom traywhich are located in the column mid-section, and the line for feedingthe salt solution from the preheater-condenser to the column mid-sectionenters the mid-section at any tray above the bottom tray and below thetop tray.
 4. The apparatus of claim 3 wherein the collecting means is acollection pan positioned below the bottom tray and the injection meansis a liquid line which leads from the collection pan to therecirculating loop.